The present invention relates to steam turbines and, more particularly, to diaphragms containing blades which direct and accelerate steam for impingement on buckets of a steam turbine.
A steam turbine conventionally employs nozzles formed by a plurality of stationary blades in the steam path which are aerodynamically shaped to receive the steam, smoothly turn it in a desired direction and accelerate it for impingement on turbine buckets. Precision in the steam path is critical to turbine efficiency. The steam must be precisely directed using diaphragm blades which are accurately figured and stably supported to avoid power wasting turbulence or off-design flow characteristics.
A diaphragm of a steam turbine is conventionally formed by inserting opposed ends of blades into cutouts in semi-circular bands known as spacers. The ends of the blades are tack welded to the spacers and outer and inner semi-circular rings are penetration welded to the spacers and to the blades. In order to obtain satisfactory attachment of the blades to the rings, very deep welding is necessary between the rings and spacers so that the weld penetrates far enough in this interface to also contact and attach the blades.
As noted in U.S. Pat. No. 4,288,677, welding defects increase as the weld depth increases. Such welding defects may include, for example, cracks, slag inclusions, lack of complete penetration, etc., which may lead to failure or instability of the nozzle blades. Furthermore, such deep welding tends to distort and to thereby deviate the steam path from the design aerodynamic characteristics and thus produce reduced overall efficiency of the apparatus.
The spacers have typically been relatively thin such as, for example, less than one-quarter inch, since this thickness was the maximum which conventional punching techniques could accommodate. Due to this thinness, the spacers themselves were effective more as positioning agents prior to welding to the inner and outer rings rather than structural members capable of supporting the blades on their own.
A further source of distortion arose due to the fabrication of spacers and blades into semi-circular assemblies. Distortions due to heating and weld shrinkage, material stress relief and tempering tended to distort this subassembly both prior to, and during, mating with the inner and outer semi-circular rings. This distortion could be traced at least in part to the fact that changing stresses in a semi-circular assembly can result in distortion of the semi-circle.
Conventional fabrication techniques have employed a radial mating plane for mating the two halves of the diaphragm. This has customarily required cutting a nozzle blade at each end of each of the semi-circular assemblies due to the overlap of the nozzle blades in the tangential direction. This resulted in relatively flexible blade portions in the steam path which could distort and vibrate to reduce steam flow aerodynamic efficiency.